We estimate the brightnesses of comets from
magnitude parameters published by the
BAA Comet Section, where these are available. These are computed
from the observations they receive from amateur astronomers.

Comets are intrinsically highly unpredictable objects, since their brightness depends on the scattering of
sunlight from dust particles in the comet's coma and tail. This dust is continually streaming away from the
comet's nucleus, and its density at any particular time is governed by the rate of sublimation of the ice in
the comet's nucleus, as it is heated by the Sun's rays. It also depends on the amount of dust that is mixed in
with that ice. This is very difficult to predict in advance, and can be highly variable even between successive
apparitions of the same comet.

In consequence, while the future positions of comets are usually known with a high degree of confidence,
their future brightnesses are not. For most comets, we do not publish any magnitude estimates at all. For
the few comets where we do make estimates, we generally prefer the BAA's magnitude parameters to those published
by the Minor Planet Center, since they are typically updated more often.